Interpretive Summary: This manuscript describes the results of an experiment on the alteration of biochar’s soil greenhouse gas impact as a result of field weathering. These particular observations have important implications in terms of documenting the time period for the suppression of N2O production following biochar amendments to soils. From this data, the impacts appear to be short term, since the weathered biochar did not possess the property of this N2O suppression. Furthermore, there was an increased rate of mineralization of the biochar observed in the soil incubations due to the field weathering. However, further research is needed to fully elucidate the importance of these initial observations and whether or not these are universal effects. These findings could provide additional insight and direction in the evaluation of biochar as a soil amendment. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved biochars based on properties to minimize greenhouse gas implications and improve soil carbon management.

Technical Abstract:
Recent observations of decreased greenhouse gas (GHG) production from biochar amended soils have been used to further substantiate the environmental benefit of biochar production and soil incorporation strategies. However, the mechanisms behind the “biochar effect” have not been fully elucidated. In addition, the duration of these GHG reductions are not known and is of pivotal importance for the inclusion of biochar into future bioenergy production and climate abatement strategies. In the present work, the impacts of biochar field aging on the observed GHG production/consumption were evaluated. Two different wood derived biochars and a macadamia nut shell biochar were weathered in an agricultural field in Rosemount, MN (2008-2011) and the impacts on net soil GHG production/consumption were assessed through laboratory incubations. For the three biochars evaluated here, weathering negated the suppression of N2O production that was originally observed from the fresh biochar in laboratory incubations. On the other hand, all three weathered biochars had enhanced CO2 production (3-10 fold compared to the fresh biochar amendments) in laboratory soil incubations, suggesting an enhanced microbial mineralization rate for the weathered biochar, which could be aided by the oxidation of the biochar surface. Fresh biochar reduced observed soil methane oxidation rates, while the weathered biochars had no significant impacts on the observed soil methanotrophic activity. This study demonstrates that for these three biochars weathering greatly alters the GHG response of the soil systems to biochar amendments.